There are many techniques that can be used for the fine pitch solder bumping process for so-called “flip chip” technology, including, for example, evaporation, electroplating, screen printing, ball drop, C4NP (Controlled Collapse Chip Connection—New Process), and so on.
As smaller solder bump interconnects, at finer pitch, are in great demand for flip chip technology, the ball drop method and C4NP, which eliminate the volume reduction problem, and which have solder alloy flexibility, have recently gained visibility and attention in the industry. Around 50% volume reduction between solder pastes and final solder bumps after reflow makes it difficult to apply the screen printing method for fine pitch applications. The electric current induced composition control can not handle wide alloy range in electroplating method. The ball drop method and C4NP allow finer pitch and a larger number of pins than the screen printing method, and they also allow more freedom in selecting the composition of solder bumps, in comparison to the electroplating method. Note that “pitch” refers to the distance between the centers of adjacent solder balls.
Since C4NP uses a solder transfer method from a glass mold to a silicon wafer after injecting and solidification of molten solder in the cavities of a glass mold, when considering the use of high melting temperature solders such as 97 weight % Pb—3 weight % Sn and 80 weight % Au—20 weight % Sn, there currently does not exist a suitable material for sealing the contact between an injection molded solder (IMS) head and a glass mold, because the temperature of the molten solder is too high when using high temperature solders.
Inoue et al., in U.S. Pat. No. 6,213,386, disclose a method of forming bumps. Solder balls and a tool having a large number of through-holes are used, and under the condition that the through-holes of the tool are aligned with the pads of the semiconductor device, the solder balls are charged into the through-holes, pressed to be fixed on the pads, and then reflowed to form bumps.
Inoue et al., in U.S. Pat. No. 6,460,755, disclose a bump forming method and an apparatus therefor. In particular, it is stated that a solder bump forming method and an apparatus therefor achieve high reliability, and an electronic part, produced by this method and this apparatus, is also disclosed. For each of the step of arraying solder balls, the step of supplying a flux, and the step of mounting the solder balls on a board, it is checked whether or not any solder ball is omitted, and the process is conducted while confirming the condition of the operation, thereby enhancing the reliability and also preventing defective products from being produced.
Shimokawa et al., in U.S. Pat. No. 6,571,007, disclose a ball-arranging substrate for forming bump, ball-arranging head, ball-arranging device, and ball-arranging method. In particular, a ball-arranging substrate comprising a substrate with a main surface, a plurality of ball-arranging holes formed on the main surface for sucking and holding minute electroconductive balls at the locations corresponding to those of electrodes formed on a semiconductor device or a printed circuit board, wherein when light illuminates the ball-arranging surface to allow optical recognition of the arrangement of the minute electroconductive balls by means of the light reflected by the minute electroconductive balls and by the main surface, the wave length of the light of the light source is set in the range of 300 to 900 nm, and the reflectivity is made not more than 50% based on the light source. A reflective mirror should be provided on the rear surface of the substrate opposite to the light source, in the case when the substrate is transparent to the irradiated light.
Bolde, in U.S. Pat. No. 6,745,450, discloses a method for loading solder balls in a mold. Solder balls are loaded into a reservoir having multiple exit ports. A removable mold is fitted into the apparatus and the reservoir is passed across the top of the mold while solder balls are fed into cavities in the mold. After the reservoir has advanced across the mold and the mold cavities are filled with solder balls, the reservoir is reset as a roller is simultaneously guided across the mold to seat the solder balls firmly within the mold. Alternatively, the roller may be applied to the solder balls while the reservoir advances across the mold, or both as the reservoir is advanced and when it is returned to its original position.
Takahashi et al., in U.S. Pat. No. 5,976,965, disclose a method for arranging minute metallic balls. In particular, a method for arranging metallic balls to form an array of bump electrodes comprises the steps of immersing a silicon template in ethanol and dropping metallic balls through the ethanol onto the template to receive the metallic balls in the holes of the template. The metallic balls are free from cohesion caused by electrostatic charge or moisture. The template may be inclined in the ethanol. The holes are formed by anisotropic etching a silicon plate.
Kuramoto et al., in U.S. Pat. No. 6,919,634, disclose a solder ball assembly, a method for its manufacture, and a method of forming solder bumps. In particular, a solder ball assembly includes a mask having first and second sides and a plurality of holes formed therein. Each hole has a first end opening onto the first side of the mask and a second end. A plurality of solder balls are disposed in the holes, and a fixing agent secures the solder balls in the holes. A protective sheet may be attached to one or both sides of the mask to cover the ends of the holes.
Kirby et al., in U.S. Pat. No. 5,431,332, disclose a method and apparatus for solder sphere placement using an air knife. A station in a manufacturing line for the accurate placement of solder balls on a ball grid array package and for the removal of excess solder balls comprises a substrate having an array of solder pads, and an adhesion layer on the solder pads. The station further comprises a stencil placed on top of the substrate and having a height between ¼ times the diameter of one of the solder balls and 5/4 times the diameter of one of the balls, the stencil having an array of apertures corresponding to the array of solder pads and substantially exposing each of the solder pads of the array, a pallet for holding and transporting the substrate to the stencil and further along the manufacturing line, a dispenser for pouring solder balls in bulk over the stencil, a vibration device coupled to the station for urging the solder balls into the apertures of the stencil and onto the adhesion layer above the solder pads, and a moving directed column of air across the surface of the stencil to remove excess solder balls from the stencil.
U.S. Pat. No. 5,985,694 of Cho discloses a semiconductor die bumping method utilizing vacuum stencil. U.S. Pat. No. 5,284,287 of Wilson et al. discloses a method for attaching conductive balls to a substrate. U.S. Pat. No. 5,540,377 of Ito discloses a solder ball placement machine.